bitbake-layers: Add ability to update the reference of repositories

bitbake-layers: Add ability to update the reference of repositories

This creates a new layers setup with, or, modifies an existing layers
setup using, one or more repositories where the references are provided
by the user.

This is a very minimal implementation, no validation of any reference
is done and it is left to the user to provide a valid value.

Signed-off-by: Jermain Horsman <jermain.horsman@nedap.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>